For many years, heat-sensitive imaging sheets have been used for copying, thermal printing, thermal recording and thermal labelling. More recently, the development of scribing lasers has enabled the use of thermally-sensitive imaging materials for the coding and marking of both sheet materials and shaped objects that may or may not be self-supporting.
Two classes of colour-forming reactants have commonly been used for thermographic materials, i.e. leuco lactone or spiropyran compounds normally developed by phenolic compounds, e.g. as described in U.S. Pat. No. 3,846,153, and heavy metal salts of organic acids that can react with ligands to give coloured complexes, e.g. as described in U.S. Pat. No. 2,663,654. The use of both these types of compounds depends on effecting a physical separation of the solid components, through dispersing them in a polymer binder, coating them on a suitable support, and melting at least one of them to cause colour formation. When coated and dried, dispersions of solid materials, by their nature, result in layers of some opacity. This is normally acceptable on opaque substrates such as paper, but limits applications on transparent substrates such as clear Mylar (polyester) film and transparent packaging films. Examples of such applications are where a film transmission original is required or, in transparent film packaging applications, where film opacity would obscure sight of the packaging contents or container surface.
There is therefore a need for transparent, thermally-sensitive imaging layers for coating on transparent or semi-transparent film supports and reflective supports such as can-metal. Further, there is a need for transparent laser-sensitive imaging materials that may be coated or printed on shaped or formed objects such as bottles and other containers for labelling or coding applications. Naturally, for these applications, the coatings should adhere to the substrate firmly and be robust, i.e. have good resistance to the types of chemical and physical treatment encountered in the end use environment. In general, organic solvent-based compositions containing solvent-soluble binders give, on drying, tougher, better adhering layers of greater transparency and water-resistance than like water-based compositions.
The use of organic amine molybdates in thermal imaging layers is described in U.S. Pat. No. 2,910,377 (see Example 10) and U.S. Pat. No. 3,028,255 (where the exemplified amines are primary amines). This use is confined to copy paper sheets, and the molybdate is dispersed by prolonged ball-milling in a resinous binder to give a suspension, used for coating. Such a suspension, when coated and dried on a transparent film support, would cause loss of transparency.
U.S. Pat. No. 4,217,409 (see Examples 10 and 12) describes the use of isopropylammonium molybdate in an acidic aqueous solution of polyvinyl alcohol as a coating that, when applied to a substrate, gives a laminar material sensitive to electromagnetic radiation including IR, visible and UV radiation. Polyvinyl alcohol solutions often have poor coating properties towards polyester film and the hazy dried films detach readily. The dried and imaged coating would also be susceptible to physical and chemical damage, most notably chemical damage from water. Isopropylamine is volatile and would cause odour should the material be contacted with aqueous alkali.
U.S. Pat. No. 4,406,839 describes the synthesis of organic solvent-soluble amine molybdates useful as smoke retardants and made from a variety of amines. Examples employ high molecular weight amines such as tridodecylamine.
Amine molybdates, their synthesis and uses, are also described in U.S. Pat. No. 2,910,377, U.S. Pat. No. 3,028,255, U.S. Pat. No. 3,290,245, U.S. Pat. No. 4,053,455, U.S. Pat. No. 4,153,792, U.S. Pat. No. 4,217,292, U.S. Pat. No. 4,217,409, U.S. Pat. No. 4,226,987, U.S. Pat. No. 4,266,051, U.S. Pat. No. 4,406,837, U.S. Pat. No. 4,406,838, U.S. Pat. No. 4,406,839, U.S. Pat. No. 4,406,840, U.S. Pat. No. 4,410,462, U.S. Pat. No. 4,410,463, U.S. Pat. No. 4,424,164, U.S. Pat. No. 4,425,279, U.S. Pat. No. 6,217,797 and U.S. Pat. No. 6,355,277.